OS .15 blue head. Great engine except for one thing, that head. Do yourself a favor and get an O'Donnell head for that thing. You will be able to lean it out quite a bit more than you have run it before. Cools much better.
Man, looking at that truck brings back some memories right there. I still have my first chassis out in my "Hole" that I used to race. Bent the crap out of that thing in a few bad crashes lol. Can't wait to see some video of this thing tearing up the grass/dirt!!